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Registered Member #49
Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362
I'm doing a RC project, and need a reciever/transmitter pair. I'm kinda awash with schematics here from Google, and need some more real-world input before I can choose one. Here are my concerns, requirements:
This is for a remote control vehicle, so I need ~1000 feet of range. What power level does this put me in?
I guess I'll be legal and aim for the RC bands, 75.idunno mhz. Here where my confusion starts. I know from traditional RC equipment that the channels are spaced by like .01Mhz (or is it .1?). If I go with a transmitter design that doesn't use a crystal, will I be able to tune my equally ghetto reciever with that precision? I don't care if my transmitter interferres with other channels, as I won't be using this around other RC'ers, but I do worry about my frequency shifting enough during playtime that my reciever loses the signal.
My signal to be transmitted is simple audio. I already tested my system with one of those 'iPod' FM transmitters and a $5.00 FM radio walkman from Walmart... the only problem was the poor range, and the illegality of using those freqs.
bottom line: can i make one of the 'simple' FM transmitter schematics on the web work reliably in the 1000 ft range, and not lose the signal? if so (please god), what are the differences between the 1, 2, 4, 1000 transistor designs out there?
Registered Member #229
Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506
I dont know much about the subject, but you can increase the distance range by increasing the power of the final emission stage (practically an amp between the station and antenna). The difference between channels is 10 kHz indeed. If you increase the emission power by N times, the distance range increase with sqrt(N) and if you increase the sensibility of your receiver of M times, the distance is bigger M times than before. You need a stable frequency though (cristal), so it is better to get a comercial one, and just add an amp to get more power emission.
Registered Member #71
Joined: Thu Feb 09 2006, 08:23AM
Location:
Posts: 63
1000 feet is no trouble at all with most of those 3-4 transistor fm xmitters out there on the net. However, the range really depends on the physical objects between the xmitter and receiver.
I once had a small 0.5mW matchbox sized xmitter kit that easily made 200m in open space, but I couldn't get any decent reception behind a tree even 5m away.
Also, what antenna do you intend on using? If you can have a directional antenna, this would be a huge advantage. This opens up the possibility of using high gain yagi's with many elements to very easily achieve your desired range with minimal effort. Yagi antenna designs are literally crawling everwhere on the net.
If you want the antenna to be omnidirectional, then something like a ground plane or simply a stiff piece of wire cut to half the xmission wavelength would work.
I remember Dave Marshall provided me with tons of valuable info on the old forum which is how I got into amatuer radio, perhaps you could PM him.
Registered Member #128
Joined: Fri Feb 10 2006, 05:02PM
Location:
Posts: 19
I am not sure how much power you will need; however, if your FM reciever uses a PLL for demodulation, the drift shouldn't matter too much as long as it stays within the capture/lock range. Just make sure your encoding scheme is tolerant of minor DC offsets.
Edit:
That is unless your reciever front end is narrow band.
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FM Trans/Reciever help
